(i) Field of the Invention
The present invention has for its object an electrochemical cell comprising a solid electrolyte, an anode and a cathode as well as the use of this electrochemical cell with a view towards the electrochemical separation of gases from a mixture of at least one gas and of oxygen or with a view towards the extraction of oxygen from a molecule containing oxygen, such as CO, CO.sub.2 and H.sub.2 O.
(ii) Description of Related Art
It is known that some solid electrolytes, comprised for example of derivatives of oxides of zirconium, yttrium, bismuth or cerium, containing or not containing dopants such as ytterbium or calcium, can be used as conductors of O.sup.2- ions, when they are subjected to an electric field and/or a difference of partial pressure of oxygen. These derivatives typically posses the same base structure derived from the so-called structure of the fluorine type presenting oxygenated gaps. They permit a conduction of O.sup.2- ions which is essentially tri-dimensional.
Besides, solid electrolyte conductors for O.sup.2- anions have been described in U.S. Pat. No. 5,227,257 the teachings of which are totally incorporated herein by reference. These electrolytes are comprised of a composition derived from Bi.sub.4 V.sub.2 O.sub.11, of which at least one of the constituent elements is substituted by one or several substituting elements chosen of the kind such that the structural type of the gama phase of Bi.sub.4 V.sub.2 O.sub.11 is maintained as well as the equilibrium of the charges. This type of solid electrolyte presents a lamellar structure in which the mechanism of conduction of the O.sup.2- ions is essentially bi-dimensional. Thus, the compositions derived from Bi.sub.4 V.sub.2 O.sub.11 are also distinguished from those typically used in the making up of solid electrolytes by their chemical nature, their crystalline structure and their mode of conducting O.sup.2- ions.
Some solid electrolytes comprised of compositions derived from Bi.sub.4 V.sub.2 O.sub.11 possess the remarkable property of permitting an artionic conductivity of 10.sup.-3 .OMEGA..sup.-1 cm.sup.-1 to 200.degree. C., which is of the order of one hundred times superior to the performances of materials currently on the market. Such classical materials are themselves inoperative at temperatures less than 300.degree. C. In order to attain an artionic conductivity on the order of 10.sup.-3 .OMEGA..sup.-1 cm.sup.-1, it is necessary to take them to temperatures greater than about 600.degree. C.
It has, however, been observed by the applicants that some classical electrodes, notably those of silver base, deposited on the electrolyte in the form of a lacquer for example, lead when they are associated with a solid electrolyte comprised of a composition derived of Bi.sub.4 V.sub.2 O.sub.11, to a rapid deactivation of the electrochemical cell. Such a deactivation is not produced when the solid electrolyte is comprised, for example, of a classical stabilized zirconium.
It is well understood that such a deactivation of the electrochemical cell is not compatible with exploitation on an industrial scale. Without being bound to a theoretical explanation, the applicants have attributed such deactivation to a chemical reaction between one or several of the constituent elements of the electrolyte derived from Bi.sub.4 V.sub.2 O.sub.11 and the electrode of silver base.
Besides, it has been observed that some electrochemical cells formed from a solid electrolyte comprised of a composition derived from Bi.sub.4 V.sub.2 O.sub.11 and of classical electrodes, when they are employed with a view towards the separation of oxygen from air, do not permit, or very weakly, such a separation.